This invention relates generally to color television receiver chrominance channels. It is more particularly related to automatic color level systems for use within such chrominance channels.
The color television broadcast signal contains components of luminance, sound, deflection synchronization, and chrominance. The chrominance components include side bands resulting from suppressed carrier modulation of a 3.58MHz chrominance sub-carrier and a reference burst. The color saturation information is represented by the amplitude of the side bands while hue information is conveyed by their resultant phase angle relative to color burst. The burst signal comprises a short duration (typically 8 to 10 cycles) sample of the suppressed chrominance carrier used in signal modulation and provides a phase and amplitude reference for chrominance demodulation and gain control.
Color television chrominance channels vary greatly in system implementation. However, all may be generally said to include a plurality of precise bandwidth amplifiers which increase the strength of the chrominance signal to a level sufficient to permit synchronous demodulation. Generally the first chrominance amplifier is provided with an automatic chroma control (ACC) which usually includes an automatic gain control loop, operative on the first chrominance amplifier, which maintains a substantially constant chrominance signal output level. In addition it is common practice to include a viewer operable gain control at some point within the chrominance channel after the ACC responsive stage for viewer adjustment of color saturation. The ACC is responsive to the amplitude of color burst signal and maintains the appropriate signal gain for "correct" signal level to the extent that a predetermined amplitude relationship between color burst and the remainder of the chrominance signal is maintained.
As is well-known in the art however, transmission inconsistencies and receiver difficulties often produce variations in the amplitude relationship between the reference burst signal and the chrominance information. This malady manifests itself undesirably in the displayed image at one extreme as over saturated noisy colors and at the other extreme as a "washed out" or nearly colorless image. These problems and the inability of ACC systems to correct them have lead practitioners in the color television art to develop an additional type of color control system generally known as automatic color level (ACL) or also referred to as automatic saturation control (ASC). The primary objective of such systems is to minimize objectionable color saturation variations at some "tolerable" distortion of the overall performance of the chrominance channel.
An automatic color level system generally comprises a detector responsive to the amplitude of the chrominance information side bands (rather than color burst) which produces a control voltage used to control the gain of one or more of the chrominance amplifiers. Because the signals forming the chrominance side bands are varied in amplitude and resulting phase, as a function of transmitted scene content, meaningful detection thereof is virtually impossible in the classic sense and therefore several "compromise" detection schemes are used. Perhaps the most common type of ACL system is that which uses an average detector, that is, one responsive to the total, or long-term average, color saturation signal level of the side bands. Because such average detection systems respond to the total color information of the system they often produce undesired attenuation of many low saturation scene components or undesired oversaturation of high scene components due to the influence of total scene composition.
A somewhat different approach involves the use of a peak, rather than average, detector. Unlike the above-mentioned system, those using a peak detector respond solely to signals exceeding a predetermined threshold and limit or reduce chrominance gain whenever such threshold is exceeded. While peak responsive systems avoid several of the difficulties caused by average systems, they also produce undesired changes in the displayed image. For example, typical peak systems respond to a small area of a high saturation within an otherwise low saturation area by undesirably reducing the entire chrominance level of both high and low saturation portions of the scene. As a result, a "washed out" or "colorless" image is produced. Some improvement has been obtained by the use of ACL systems which are responsive in some measure to both peak and average detectors. While such systems are in many cases more desirable than either average or peak systems, their total performance nontheless produces an often undesirable compromise in color saturation correction and may become prohibitively high in cost.
Accordingly, it is a general object of the present invention to provide an improved automatic color level system. It is a more particular object of the present invention to provide an improved automatic color level system which provides saturation changes of each scene element independent of other scene elements.